Expandable interbody fusion cage

ABSTRACT

An expandable intervertebral device including a body extending generally along a longitudinal axis and including at least two branch portions coupled together adjacent an end portion of the body. The device further includes an expansion member positioned between the at least two branch portions with at least a portion of the expansion member arranged at an angular orientation relative to the longitudinal axis whereby a change in the angular orientation relative to the longitudinal axis urges the at least two branch portions apart to expand the body.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present Application is a continuation of U.S. applicationSer. No. 09/949,516, filed Sep. 7, 2001, which is a continuation of U.S.application Ser. No. 09/763,073, filed Aug. 26, 1999 and issued as U.S.Pat. No. 6,436,140, which claims foreign priority benefits ofInternational Patent Application Number PCT/IB99/01478, filed Aug. 26,1999, and French Patent Application Number FR98/10832, filed on Aug. 28,1998, the contents of each application hereby being incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an implantable device forpromoting fusion between two adjacent bony structures and a method ofinserting the same. More particularly, the invention relates to anexpandable fusion cage that may be inserted, in a reduced sizeconfiguration, into an intervertebral space and expanded after insertionto provide a desired size. While the device according to the presentinvention may have application in other areas of the body, the presentinvention is preferably utilized for vertebral interbody fusion.

[0003] There have been an extensive number of attempts to develop anexceptional intradiscal implant that could be used to replace a damageddisc and yet maintain the stability of the disc interspace betweenadjacent vertebra, at least until complete arthrodesis is achieved.These “interbody fusion devices” have taken many forms. For example, oneof the more prevalent designs takes the form of a cylindrical implant.These types of implants are presented by the patents to Bagby, U.S. Pat.No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos.4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In thecylindrical implants, the exterior portion of the cylinder can bethreaded to facilitate insertion of the interbody fusion device, asrepresented by the Ray, Brantigan and Michelson patents. In thealternative, some of the fusion implants are designed to be pounded intothe intradiscal space. This type of device is represented by the patentto Brantigan.

[0004] Interbody fusion devices can be generally divided into two basiccategories, namely solid implants and implants that are designed topermit bone in-growth. Solid implants are represented by U.S. Pat. Nos.4,879,915; 4,743,256; 4,349,921; and 4,714,469. The remaining patentsdiscussed above include some aspect that allows bone to grow across theimplant. It has been found that the devices that promote natural bonein-growth achieve a more rapid and stable arthrodesis. The devicedepicted in the Michelson patent is representative of this type ofhollow implant which is typically filled with a bone growth inducingsubstance to promote bone growth into and through the device. Thisimplant includes a plurality of circular apertures which communicatewith the hollow interior of the implant, thereby providing a path fortissue growth between the vertebral end plates and the bone growthmaterial within the implant. In preparing the intradiscal space, the endplates are preferably reduced to bleeding bone to facilitate the tissuein-growth. During fusion, the metal structure provided by the Michelsonimplant helps maintain the patency and stability of the motion segmentto be fused. In addition, once arthrodesis occurs, the implant itselfserves as a sort of anchor for the solid bony mass.

[0005] One problem that is not addressed by the above prior devicesconcerns maintaining and restoring the normal anatomy of the fusedspinal segment. Naturally, once the disc is removed, the normal lordoticor kyphotic curvature of the spine is eliminated. With the priordevices, the need to restore this curvature is neglected. For example,adjacent vertebral bodies may be reamed with a cylindrical reamer thatfits the particularly implant. In some cases, the normal curvature isestablished prior to reaming and then the implant inserted. However,this over-reaming of the posterior portion is generally not wellaccepted because of the removal of load bearing bone of the vertebraeand because it is typically difficult to ream through the posteriorportion of the lower lumbar segment where the lordosis is the greatest.In most cases using implants of this type, no effort is made to restorethe lordotic curvature so that the cylindrical implant is likely tocause a kyphotic deformity as the vertebrae settles around the implant.This phenomena can often lead to revision surgeries because the spinebecomes imbalanced.

[0006] In each of the above-listed patents, the transverse cross-sectionof the implant is substantially constant throughout its length and istypically in the form of a right circular cylinder. Other implants havebeen developed for interbody fusion that do not have a constantcross-section. For instance, the patent to McKenna, U.S. Pat. No.4,714,469 shows a hemispherical implant with elongated protruberancesthat project into the vertebral end plate. Further, U.S. Pat. No.5,669,909 to Zdeblick et al., shows a truncated conical implant adaptedto be threadedly received in the intervertebral space. However, thesedevices require an opening at least as large as the largest segment ofthe device. The requirement for such a relatively large opening maylimit the use of such devices, particularly where access to the spine islimited due to obstructing vessels and neurological structures.

[0007] Still further implants have been developed that provide theability to adjust the size of the implant after insertion. U.S. Pat. No.5,665,122 to Kambin, U.S. Pat. No. 5,554,191 to LaHille et al., and U.S.Pat. No. 5,653,763 to Errico et al. disclose implants which provide atleast some degree of adjustability of the height of the implant torestore lordosis. However, these implants do not allow the device to beeasily and securely inserted into a disc space and the internalexpansion mechanism limits the ability to pack the interior with a largeamount of bone in-growth material.

[0008] In view of the limitations of the prior devices, there remains aneed for an expandable interbody device capable of stabilizing the spinein a manner comparable to interbody implant designs presently in use,and at the same time providing a mechanism for restoring normal lordosisof the spine. After expansion, the device should have an internal cavityadapted to receive bone graft or bone substitute to encourage bonegrowth through the expanded device.

SUMMARY OF THE INVENTION

[0009] In response to the needs still left unresolved by the priordevices, the present invention contemplates an expandable intervertebraldevice adapted to be inserted between a pair of vertebral bodies torestore the normal angular relation between adjacent vertebrae. Inparticular, an expandable intervertebral device according to one form ofthe present invention includes a body extending generally along alongitudinal axis and including at least two branch portions coupledtogether adjacent an end portion of the body. The device furtherincludes an expansion member positioned between the at least two branchportions with at least a portion of the expansion member arranged at anangular orientation relative to the longitudinal axis whereby a changein the angular orientation relative to the longitudinal axis urges theat least two branch portions apart to expand the body.

[0010] In another form of the present invention, an expandableintervertebral device is provided including a body extending generallyalong a longitudinal axis and including at least two branch portionscoupled together adjacent an end portion of the body, with the bodydefining a series of ratchet elements positioned along the longitudinalaxis. The device further includes an expansion member positioned betweenthe at least two branch portions whereby axial movement of the expansionmember generally along the longitudinal axis urges the at least twobranch portions apart to expand the body with a portion of the expansionmember engaged with at least one of the ratchet elements to maintain theexpansion member in a select axial position along the longitudinal axisrelative to the body.

[0011] In another form of the present invention, an expandableintervertebral device is provided including a body extending generallyalong a longitudinal axis and having a fixed end portion and a movableend portion, with the body including at least two branch portionscoupled together adjacent the fixed end portion. The device furtherincludes an expansion member positioned adjacent the movable end portionof the body whereby axial movement of the expansion member from themovable end portion toward the fixed end portion results in engagementbetween the expansion member and the at least two branch portions tourge the at least two branch portions apart to expand the body.

[0012] In other forms of the present invention, methods are provided forthe insertion of an expandable intervertebral device between an adjacentpair of vertebrae.

[0013] One object of the present invention is to provide an expandableintervertebral device that has a reduced size insertion configurationand is expandable from the insertion configuration to a largerconfiguration.

[0014] Another object of the present invention is to provide anexpandable intervertebral device that has a substantially unobstructedinterior chamber to receive bone growth promoting material.

[0015] Still another object of the present invention is to provide anexpandable intervertebral device configured for easy insertion andexpandable to a larger size to establish lordosis.

[0016] Yet a further object of the present invention is to provide animproved method for inserting an expandable intervertebral device into adisc space to restore lordosis.

[0017] Related objects and advantages of the present invention will beapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a top view of an expandable cage according to oneembodiment of the present invention.

[0019]FIG. 2a is a side cross-sectional view of the device of FIG. 1.

[0020]FIG. 2b is the device of FIG. 2a with the inclusion of anexpansion wedge according to one embodiment of the present invention.

[0021]FIG. 3 is a partial cross-sectional perspective view of theexpandable cage of FIG. 1 without an external thread pattern.

[0022]FIG. 4 is a perspective view of the expansion wedge of FIG. 2b.

[0023]FIG. 5 is an end view of the expansion wedge of FIG. 4.

[0024]FIG. 6 is an elevational view of an insertion tool according toone embodiment of the present invention.

[0025]FIG. 7a is a sagittal plane view showing a partial cross-sectionalside view of the expandable cage of FIG. 2b inserted between twoadjacent vertebrae in an insertion configuration according to oneembodiment of the present invention.

[0026]FIG. 7b is the cage of FIG. 7a shown in an expanded positionaccording to one embodiment of the present invention.

[0027]FIG. 8 is a top view of an alternative embodiment of theexpandable cage of FIG. 1.

[0028]FIG. 9 is a side cross-sectional view of the expandable cage ofFIG. 8.

[0029]FIG. 10 is a partial cross-sectional perspective view of theexpandable cage of FIG. 8 without an external thread pattern.

[0030]FIG. 11 is a top view of a further embodiment of an expandablecage according to the present invention.

[0031]FIG. 12 is a side partial cross-sectional view of the expandablecage of FIG. 11.

[0032]FIG. 13 is a partial cross-sectional perspective view of theexpandable cage of FIG. 11, without an external thread pattern.

[0033]FIG. 14 is a side partial cross-sectional view of a furtherembodiment of the present invention.

[0034]FIG. 15 is a side partial cross-sectional side view of yet afurther embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated devices, and such furtherapplications of the principles of the invention as illustrated thereinbeing contemplated as would normally occur to one skilled in the art towhich the invention relates.

[0036] Referring now to FIGS. 1 through 3, there is shown a firstembodiment of an expandable cage 10 in accordance with the presentinvention. In this embodiment of the invention, expandable cage 10 has acylindrical outer surface 11 defining an external thread pattern 12 (notshown in FIG. 3) adapted to engage two adjacent vertebra (see FIGS. 7aand 7 b) and to advance the cage into the disk space as cage 10 isrotated about longitudinal axis 13. As shown most clearly in FIG. 3,expandable cage 10 is substantially hollow with inner surface 21defining an internal cavity 17. Expandable cage 10 includes a bonein-growth window 16 formed through expandable branch 24 and an identicalbone in-growth window 19 formed through expandable branch 26. Thesein-growth windows are adapted to permit communication between thevertebral bodies (FIG. 7) and internal chamber 17. In most application,bone growth promoting material will be placed within internal chamber 17of expandable cage 10 to encourage bone to grow into and through fusiondevice 10.

[0037] As shown in the accompanying drawings, expandable cage 10 ispreferably composed of four separate branches, each separated atexpandable end 18 by a channel extending longitudinally from expandableend 18 toward fixed end 20. Referring now to FIG. 2a, first expandablebranch 24 is separated from first fixed branch 40 by channel 14. In asimilar manner, second expandable branch 26 is separated from firstfixed branch 40 by channel 22. Each of channels 14 and 22 extends fromexpandable end 18 towards fixed end 20. The channels terminate in aslightly larger diameter radiused opening which preferably acts as ahinge during expansion of the device to concentrate stress anddeformation adjacent fixed end 20. In a similar fashion, as shown inFIG. 3, second fixed branch 41 is separated from first expandable branch24 by channel 15 and from second expandable branch 26 by a similarchannel (not shown). Thus, expandable cage 10 is formed by fourbranches, each separated from the other at expandable end 18 by channelsextending from outer surface 11 to internal chamber 17. The branches areconnected at fixed end 20 by linking area 44 such that each of thebranches may move substantially independent from each other atexpandable end 18 while remaining connected to the device by linkingarea 44. Although the present embodiment utilizes integrally formedbranches, it is contemplated that separate components may be joined toform the expandable cage without deviating from the invention.

[0038] While four separate branches are shown in a preferred embodiment,it is contemplated that more or less branches could be utilized withoutdeviating from the spirit or scope of the invention. Moreover, althoughin a preferred embodiment the channels extend from outer surface 11 tointernal chamber 17, it is contemplated that the channels may not extendto the interior chamber. Such a channel may be formed by an overlappinginterface between two adjacent branches without creating an opening forbone in-growth into internal chamber 17. Further, although channels areshown as being preformed in the expandable cage, it is contemplated thatthe channels may be formed as the implant is expanded. By way ofexample, and without limitation, this could occur by rupturing afrangible portion between adjacent branches or by deforming materialdisposed between adjacent branches.

[0039] In one aspect of the invention, internal chamber 17 comprises themajority of the volume of the entire cage 10. Specifically referring toFIG. 2a, external surface 11 defines a diameter 63, excluding threads12. Internal chamber 17 has a diameter of at least 64, that is diameter63 less twice the branch thickness 62.

[0040] In a preferred embodiment, branch thickness 62 is selected suchthat the volume of internal chamber 17, particularly with the inclusionof voids created by in-growth windows 16 and 19 and the variouschannels, occupies the majority of the entire volume of cage 10. Thus,the present invention provides space for a large volume of bone growthpromoting material to be inserted into the device to promote bonein-growth.

[0041] Adjacent expansion end 18, as shown in FIG. 2a with respect tobranches 24, 40 and 26, each of the branches includes inclined surfaces28, 34 and 30, respectively. Adjacent the internal termination of eachof the inclined surfaces 28, 34 and 30, is formed an internal shoulder36, 42 and 38, respectively. Referring to FIG. 3, branch 41 includes asimilar inclined surface 35 and internal shoulder 43. As shown in FIG.2a, arcuate inclined surfaces 28 and 30 are inclined at angle 32 withrespect to each other. In a preferred embodiment, this angle isapproximately 96°, although it is understood that a variety of anglescould be utilized depending on the amount of expansion desired, and thedistance an expansion member will need to travel to create theexpansion.

[0042] Referring now to FIG. 2b, cage 10 is shown with the inclusion ofexpansion wedge 50 disposed adjacent expandable end 18. Expansion wedge50 is further illustrated in FIGS. 4 and 5. Expansion wedge 50 includesfirst and second opposed expansion wedge inclines 52 and 54, which havea tapering arcuate surface that mates with and corresponds to inclinedsurfaces 28 and 30 of branches 24 and 26, respectively. Wedge 50includes side walls 58 and 60 with substantially planar surfaces adaptedto engage inclined surfaces 34 and 35 of fixed branches 40 and 41,respectively. Further, expansion wedge 50 includes a central opening 56which is adapted to receive a driving tool. In a preferred embodiment,central opening 56 is threaded to receive a correspondingly threaded endof a driving tool.

[0043] It will be understood that as wedge 50 is advanced toward fixedend 20, inclined surfaces 52 and 54 act upon inclined surfaces 28 and30, respectively, to urge branches 24 and 26 apart from each other atexpandable end 18. As expansion wedge 50 is inserted along inclinedsurfaces 28 and 30, branches 24 and 26 will tend to expand at expansionend 18 as deformation occurs adjacent fixed end 20. Substantiallycontinuous linking material 44 links all of the branches and does notpermit their expansion at fixed end 20. As wedge 50 is further advancedtoward fixed end 20, the wedge is pushed beyond shoulders 36 and 38,such that wedge 50 is captured within cage 10. As a result of theengagement of back surface 55 of the wedge against shoulders 36 and 38,expansion wedge 50 is prevented from being expelled from the cage, andthe cage is maintained in an expanded condition with the wedge securelyheld in position. It will be understood that the engagement of planarsurfaces 58 and 60 on fixed branches 40 and 41, respectively, tends todeformably expand these branches little, if any, and therefore theyremain substantially fixed in their original positions. Further, fixedbranches 40 and 41 each include shoulders 42 and 43 which engage theback surface 55 of wedge 50 once it has passed beyond the shoulders.Thus, fixed branches 40 and 41 also tend to hold the wedge in place andprevent its expulsion from internal chamber 17.

[0044] Referring now to FIGS. 8 through 10, there is shown analternative embodiment of the expandable cage of the present invention.In this embodiment, expandable cage 310 has an outer surface 311 and anexternal thread pattern 312 (not shown in FIG. 10). As with theembodiment of FIG. 1, expandable cage 310 includes two opposedexpandable branches 324 and 324, and two opposed fixed branches 340 and341 joined at fixed end 320. Each of these branches define inclinedsurfaces and interior shoulders adjacent expandable end 318 adapted toengage and receive expansion wedge 350. In contrast to the embodiment ofFIG. 1, expandable branch 324 includes two bone in-growth windows 316and 317, separated by rib 330. In a similar manner, expandable branch326 includes two bone in-growth windows 322 and 323 separated by rib332. The use of a plurality of bone in-growth windows in the expandablebranches increases the overall strength of the branch which may benecessary for longer cages or cages constructed of relatively weakmaterials. While this embodiment has been shown with two windows pereach expandable branch, it is contemplated that more than two bonein-growth windows may be utilized without deviating from the spirit andscope of the invention.

[0045] Referring now to FIG. 6, there is shown an insertion tool 70suitable for use with an expandable cage according to the presentinvention. Insertion tool 70 includes an outer sleeve 72 having adriving projection 73 adapted to engage driving groove 46 of expandablecage 10. While insertion tool 70 is illustrated with only projection 73,it will be understood that the device includes an opposing projection(not shown) for mating with a driving groove (not shown) disposedopposite driving groove 46 on cage 10. Insertion tool 70 furtherincludes handle 71 adapted to transmit rotational force to outer sleeve72 to rotationally insert cage 10. Outer sleeve 72 includes an internalchamber, which is occupied by insertion tool inner shaft 75. On thedistal end 77, insertion tool inner shaft 75 includes an externallythreaded area adapted to engage the correspondingly internally threadedcentral opening 56 of expansion member 50. At the proximal end of innershaft 75 there is a handle 76 for providing rotational force to innershaft 75. A series of external threads 78 are formed on inner shaft 75adjacent handle 76. An internally threaded nut 79 is disposed aboutinner shaft 75 and is adapted to engage threads 78 to move inner shaft75 with respect to outer sleeve 72.

[0046] In operation, insertion tool 70 is engaged with expandable cage10 such as shown in FIG. 7a. Driving projection 73 of outer tube 72engages driving groove 46 of expandable cage 10 and threaded distal end77 of inner shaft 75 threadedly engages threaded opening 56 of expansionwedge 50. In this manner, expansion wedge 50 is securely held inposition adjacent the expansion end 18 while the threaded cylindricalcage is inserted into the intervertebral space. In a preferredembodiment, this device is utilized from a posterior approach to thespine with the expansion wedge 50 being positioned at the leading end ofthe device.

[0047] Referring to FIG. 7a, expandable cage 10 is threaded intointervertebral space 92 with thread pattern 12 engaging vertebra 80 and82 to advance the cage into the disc space and securely hold it inposition once it has reached a final position as shown in FIG. 7a. Sincethe expandable cage is preferably a cylinder having a uniform diameter,it may be inserted through an insertion tube having a diametersubstantially equal to the thread diameter of the cage 10. As shown inFIG. 7a, the surface 88 of vertebrae 80 is in contact with outer surface11 of cage 10. It will be understood that in many applications, aportion of the vertebral end plate will have been removed prior to cageinsertion such that cage 10 engages the cancellous bone of thevertebrae. In a similar manner, the surface 90 of vertebrae 82 is incontact with the outer surface 11 of cage 10. In its initial insertionposition, the alignment 84 of vertebrae 80 and the alignment 86 ofvertebrae 82 are in substantial parallel alignment with longitudinalaxis 13 and expandable branches 24 and 26 of cage 10.

[0048] Referring now to FIG. 7b, with threaded end 77 of the insertiondevice firmly engaged in threaded opening 56 of expansion wedge 50 anddriving projection 73 engaged in driving groove 46, internally threadednut 79 is rotated about external thread 78 to draw shaft 75 within outertube 72 (FIG. 6), thereby advancing expansion wedge 50 toward fixed end20. As expansion wedge 50 is advanced toward fixed end 20, the inclinedsurfaces of expansion wedge 50 force expandable branches 24 and 26 apartadjacent expandable end 18. In a similar manner, vertebra 80 and 82 areforced apart adjacent expandable end 18 such that the alignment 84 and86 remain substantially parallel to the expandable branches 24 and 26,respectively, and not with longitudinal axis 13 of cage 10. In thismanner, the lordotic curve of the spine may be established andmaintained during the healing process. Moreover, the engagement ofexpansion wedge 50 with the previously described shoulders of each ofthe branches prevents the expansion wedge from being expelled from cage10. The insertion tool may be removed and the substantially unobstructedarea within interior chamber 17 may be filled with bone in-growthmaterial to encourage bone growth through the device. Such bonein-growth material may include autograft, allograft, bone morphogenicproteins in a carrier, or other known bone growth promoting materials.Insertion and expansion of the alternative embodiment shown in FIGS. 8through 10 is accomplished in substantially the same manner.

[0049] Referring now to FIGS. 11 through 13, there is shown a furtherembodiment of an expandable cage according to the present invention. Aswith the above-described embodiments of the invention, cage 110 is asubstantially cylindrical device having an outer surface 111 defining anexternal thread pattern 112. Cage 110 defines a substantiallycylindrical internal chamber 117. Cage 110 includes a pair of opposingexpandable branches 152 and 154 separated by a pair of opposing fixedbranches 148 and 150. Fixed branch 148 is separated from expandablebranches 152 and 154 by channels 144 and 146, respectively. Fixed branch150 is separated from expandable branches 154 and 152 by channel 147 anda similar channel (not shown), respectively. Cage 110 further includesbone in-growth windows 118 and 120 formed through expandable branch 152,and an identical pair of bone in-growth windows 119 and 121 formedthrough expandable branch 154. Each of the bone in-growth windows extendfrom outer surface 111 to internal chamber 117. As with theabove-described embodiments, cage 110 includes a large unobstructedinternal chamber 117 extending along the longitudinal axis 113 fromadjacent the expandable end 118 toward the fixed end 114. In theembodiment shown in FIGS. 11 through 13, windows 120 and 121 eachinclude a notch 122 and 123 adjacent expandable end 118, respectively.

[0050] Expander 130 is sized to be received within internal chamber 117.Expander 130 includes a first portion 125 having a projection 126 whichextends into notch 122, and an opposite second portion 127 havingprojection 128 which extends into notch 123. Projections 126 and 128work in conjunction with externally threaded plug 124 in maintaining theposition of expander 130 within cage 110. Expander 130 further includesa bend 132. While a bend may be utilized in the preferred embodiment, itwill be understood that expander 130 may include a fold or a hingebetween portions 125 and 127, that allows adaptation into the reducedsized configuration shown in FIG. 12. First portion 125 includes alongitudinal axis 72 and second portion 127 includes a longitudinal axis70. In the reduced size insertion configuration shown in FIG. 12,longitudinal axis 70 forms an angle 162 with respect to longitudinalaxis 72. In a preferred embodiment, angle 162 is approximately 90°,although other angles are contemplated. In an expanded configuration(not shown), the angle between longitudinal axes 70 and 72 may approach180°, with a 180° angle providing the maximum expansion of the device.

[0051] The internal chamber 117 is defined by the four previouslydescribed branches 148, 150, 152, and 154, each defining a portion ofthread pattern 160 (only partially shown in FIG. 12). Plug 124 includesa corresponding external thread adapted to engage thread pattern 160. Ina preferred embodiment, a connecting portion 149 extends between fixedbranches 148 and 150 to limit splaying of the fixed branches as threadedplug 124 is advanced toward expandable end 118. Threaded plug 124further includes a central opening 136 adapted to engage an insertiontool extension. In a preferred embodiment, central opening 136 is formedin a hexagonal pattern to accept a similar hexagonally shaped insertiontool (not shown). Cage 110 further includes a driving groove 142adjacent fixed end 114, adapted for engagement with a driving toolprojection to permit insertion of cage 110 between two adjacent bonystructures. The driving tool of FIG. 6 may be utilized with cage 110with the exception that the driving tool inner shaft 75 would include ahexagonally shaped portion at distal end 77. It will be understood thatas threaded plug 124 is threadedly advanced towards expandable end 118,it urges expander 130 into an expanded condition, thereby forcingbranches 152 and 154 apart. As shown in FIG. 13, the expansion mechanismof the present invention provides a large internal cavity to receivebone growth promoting material.

[0052] A further embodiment according to the present invention is shownin FIG. 14. Cage 180 is fashioned in a similar manner to cage 110 withthe exception that it includes a plurality of smaller bone in-growthwindows rather than two large windows in expandable branches 192 and194. Variations of the number, size, shape and location of bonein-growth windows as may be dependent on the bone in-growthcharacteristics desired and the material properties of the cage iscontemplated by the present invention. Further, the mechanism forexpansion differs in that expander 184 is a substantially planar device,i.e. no bends or hinge, having a longitudinal axis 198. Expander 184 issubstantially planar and has a first end 188 engaged in corner 186formed between expandable branch 194 and end wall 199. The opposite end190 engages and moves along inner surface 193 of expandable branch 192.Threaded plug 182 threadedly engages internal thread pattern 196 formedalong the internal surfaces of the branches. It will be understood thatin the unexpanded condition, axis 198 is skewed with respect tolongitudinal axis 197 of cage 180. However, as threaded plug 182advances towards expandable end 181, expander 184 moves towards anupright position with axis 198 moving towards a perpendiculararrangement with axis 197. The movement of expander 184 towards anupright position expands cage 180. In the expanded position, there is alarge unobstructed internal chamber 189 extending from plug 182 toopening 187 adjacent fixed end 195. Thus, the large internal chamber 189may be packed with bone in-growth material to promote fusion betweenadjacent vertebra.

[0053] Referring now to FIG. 15, there is shown still a furtherembodiment of the present invention. Cage 210 includes an outer surface211 having a thread pattern 212 defined thereon. Cage 210 includes adriving groove 246 adapted to receive a driving tool such as thatpreviously disclosed herein. Cage 210 further includes a plurality ofwindows 214 communicating from exterior surface 211 to interior chamber217. Internal chamber 217 is defined by inclined surfaces 216 and 215(shown in dash), sloping from the expandable end 222 towards the fixedend 224. The slope of inclined surfaces 216 and 215 could also bereversed to allow expansion by movement of the plug 218 in an oppositedirection. Plug member 218 includes an external thread pattern adaptedto engage with thread pattern 220 of surfaces 216 and 215. It will beunderstood that as plug member 218 is threadedly advanced toward fixedend 224, branches 230 and 231 are spread apart from one another. Asshown in FIG. 15, branch 230 includes an area of reduced width 225,adapted to deform as plug member 218 is advanced. As shown, plug member218 includes a central opening 237 adapted to receive an insertion toolextension to provide rotational force. Further, while driving groove 246is shown formed on expandable end 222, it will be understood that forsome insertion techniques, it will be desirable to have insertion toolgroove 246 formed on fixed end 224. Moreover, a central aperture may beformed through fixed end 224 for passage of an insertion tool extensionfor engagement with plug 218.

[0054] While plugs 124 of the embodiment of FIG. 11, plug 182 of theembodiment of FIG. 14 and plug 218 of the embodiment of FIG. 15 havebeen shown and described as having a series of external threads forengagement with a corresponding thread pattern defined on the branchesof the device, it will be understood that all the branches, or only thefixed branches of each of the devices, may be formed to define a seriesof ratchets. With a ratchet configuration, each of the plugs 124, 182,and 218 may be defined as having an outer surface adapted to advanceover the ratchets to expand the device while having a trailing portionadapted to engage the ratchets to prevent expulsion. In this manner, theplugs may be advanced without threading. When utilizing this technique,the insertion tool may be adapted to securely hold the outer cage toprevent its further advancement as a result of the pushing or pullingforce exerted on the plug members. In addition to modifications to theplug, the cage itself may be configured for push-in insertion and can bein shapes other than cylinders.

[0055] Cages according to the present invention are preferably formed ofa biocompatible material having sufficient strength to withstand theloads that will be placed upon them for a given application.Additionally, in the preferred embodiments the material should havesufficient flexibility to undergo at least a small amount of deformationas a result of the expansion process. Alternatively, for some devices,it may be desirable to provide hinge points rather than permit thematerial to undergo a deformation. Most preferably, the materialutilized to form the cages of the present invention is a medical gradetitanium alloy. However, the devices could be formed of stainless steel,various types of plastic, various composites including carbon fiberdevices, and bone or bone substitutes.

[0056] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only the preferred embodiment has been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An expandable interbody device, comprising: abody extending generally along a longitudinal axis and including atleast two branch portions coupled together adjacent an end portion ofsaid body; and an expansion member positioned between said at least twobranch portions with at least a portion of said expansion memberarranged at an angular orientation relative to said longitudinal axiswhereby a change in said angular orientation relative to saidlongitudinal axis urges said at least two branch portions apart toexpand said body.
 2. The device of claim 1, wherein axial displacementof at least a portion of said expansion member generally along saidlongitudinal axis facilitates said change in said angular orientation ofsaid expansion member.
 3. The device of claim 2, further comprising anactuator member positioned adjacent said expansion member; and whereindisplacement of said actuator member generally along said longitudinalaxis results in said displacement of said at least a portion of saidexpansion member to facilitate said change in said angular orientation.4. The device of claim 3, wherein said actuator member defines externalthreads engagable along internal threads defined by said body todisplace said actuator member generally along said longitudinal axis. 5.The device of claim 1, wherein said expansion member includes a firstportion and a second portion, said first portion extending along a firstaxis and engaged with a first of said branch portions, said secondportion extending along a second axis and engaged with a second of saidbranch portions, said first axis forming an angle relative to saidsecond axis; and wherein a change in said angle between said first andsecond axes urges said first and second branch portions apart to expandsaid body.
 6. The device of claim 5, wherein said first and secondportions of said expansion member are coupled together at aninterconnection location; and wherein axial displacement of saidinterconnection location generally along said longitudinal axisfacilitates said change in said angle between said first and second axesto expand said body.
 7. The device of claim 6, further comprising anactuator member positioned adjacent said interconnection location ofsaid expansion member; and wherein displacement of said actuator membergenerally along said longitudinal axis results in said axialdisplacement of said interconnection location.
 8. The device of claim 5,wherein said expansion member includes a bend interconnecting said firstand second portions to facilitate said change in said angle between saidfirst and second axes.
 9. The device of claim 5, wherein said expansionmember includes a hinge interconnecting said first and second portionsto facilitate said change in said angle between said first and secondaxes.
 10. The device of claim 5, wherein said angle between said firstand second axes falls within a range of about 90 degrees to about 180degrees.
 11. The device of claim 1, wherein said expansion memberincludes a first end portion engaged with a first of said branchportions and a second end portion engaged with a second of said branchportions; and wherein displacement of said first end portion relative tosaid second end portion generally along said longitudinal axisfacilitates said change in said angular orientation of said expansionmember to expand said body.
 12. The device of claim 11, furthercomprising an actuator member positioned between said first and secondbranch portions; and wherein said first end portion of said expansionmember is positioned adjacent said actuator member and said second endportion of said expansion member is positioned adjacent a shoulderdefined by one of said first and second branch portions; and whereindisplacement of said actuator member generally along said longitudinalaxis results in said displacement of said first end portion relative tosaid second end portion to facilitate said change in said angularorientation of said expansion member.
 13. The device of claim 11,wherein said expansion member has a substantially planar configuration.14. The device of claim 1, wherein said at least a portion of saidexpansion member is skewed relative to said longitudinal axis when saidbody is in an initial state; and wherein said at least a portion of saidexpansion member is arranged substantially perpendicular to saidlongitudinal axis when said body is transitioned to an expanded state.15. The device of claim 1, wherein said body defines an internal chamberand a number of bone in-growth openings extending through said body incommunication with said internal chamber.
 16. The device of claim 15,further comprising a bone growth promoting material positioned withinsaid internal chamber to promote bone in-growth through said openingsand into said internal chamber.
 17. The device of claim 15, wherein atleast one of said bone in-growth openings extends through each of saidat least two branch portions in communication with said internalchamber.
 18. The device of claim 1, wherein said body has asubstantially cylindrical configuration.
 19. The device of claim 18,wherein said body defines external threads configured for engagementwith vertebral bone.
 20. An expandable interbody device, comprising: abody extending generally along a longitudinal axis and including atleast two branch portions coupled together adjacent an end portion ofsaid body, said body defining a series of ratchet elements positionedalong said longitudinal axis; and an expansion member positioned betweensaid at least two branch portions whereby axial movement of saidexpansion member generally along said longitudinal axis urges said atleast two branch portions apart to expand said body, a portion of saidexpansion member engaged with at least one of said ratchet elements tomaintain said expansion member in a select axial position along saidlongitudinal axis relative to said body.
 21. The device of claim 20,wherein said portion of said expansion member defines an outer surfaceconfigured to advance over said ratchet elements and having a trailingportion configured for engagement with said at least one of said ratchetelements to maintain said expansion member in said select axialposition.
 22. The device of claim 20, wherein said series of ratchetelements are defined by said at least two expandable branch portions.23. The device of claim 20, wherein said body has a substantiallycylindrical configuration.
 24. The device of claim 23, wherein said bodyincludes external threads configured for engagement with vertebral bone.25. An expandable interbody device, comprising: a body extendinggenerally along a longitudinal axis and having a fixed end portion and amovable end portion, said body including at least two branch portionscoupled together adjacent said fixed end portion of said body; and anexpansion member positioned adjacent said movable end portion of saidbody whereby axial movement of said expansion member from said movableend portion toward said fixed end portion results in engagement betweensaid expansion member and said at least two branch portions to urge saidat least two branch portions apart to expand said body.
 26. The deviceof claim 25, wherein at least one of said expansion member and saidmovable end portion of said body defines a tapered surface to facilitatesaid engagement between said expansion member and said at least twobranch portions to urge said at least two branch portions apart.
 27. Thedevice of claim 25, wherein said body defines a retention mechanism tomaintain said expansion member in a select position subsequent toexpansion of said body.
 28. The device of claim 27, wherein saidretention mechanism comprises an internal shoulder.
 29. The device ofclaim 25, further comprising an insertion tool having an outer sleeveremovably engaged with said body and an inner shaft moveably disposedwithin said outer sleeve and including a distal end portion removablyengaged with said expansion member, whereby axial displacement of saidinner shaft with respect to said outer sleeve results in said axialmovement of said expansion member.
 30. The apparatus of claim 29,wherein said body further defines a driving groove, said outer sleevedefining a corresponding driving projection positioned within saiddriving groove to removably engage said outer sleeve with said body.